a) Field of the Invention
The present invention relates to a zoom optical system which is to be used with photographic cameras, and lens shutter cameras in particular.
b) Description of the Prior Art
In the recent years, lens shutter cameras which are equipped with zoom optical systems are configured as the so-called collapsible mount type which accommodate lens systems into camera bodies by narrowing airspaces between adjacent lens units of the optical systems. For reducing thickness of a lens shutter camera, it is therefore necessary to shorten a total length of an optical system in its collapsed condition. Further, a zoom optical system has a long total length at its tele position, and it is necessary to configure a lens barrel which accommodates and holds a lens system so that it can be accommodated in two or more stages in a camera body for collapsing the optical system compact at its tele position, thereby reducing thickness of the camera.
A typical zoom optical system which is conventionally used in lens shutter cameras is composed of three positive, positive and negative lens units for obtaining a relatively high vari-focal ratio and a compact design.
For obtaining a wide field angle of incidence at a wide position of such a zoom optical system which is composed of the three positive, positive and negative lens units, a total refractive power of the first positive lens unit and the second positive lens unit must be strengthened at the wide position, and the second lens unit must be composed of a large number of lens elements for favorably correcting offaxial aberrations in the second lens unit in particular.
For maintaining favorable optical performance in all zoom regions from a wide position to a tele position, it is necessary to suppress amounts of aberrations to be produced in each zoom lens unit. Since the third lens unit produces aberrations in large amounts at the tele position in particular, it is necessary for reducing the aberrations to use two or more lens elements in the third lens unit.
Let us suppose that we are to obtain a zoom optical system which is composed of three positive, positive and negative lens units, covers a region wherein field angles are large and has a vari-focal ratio of 3 or higher. When a lens barrel is composed of two or three stages for collapsing the zoom optical system which has a long total length at its tele position and moves the first lens unit for a long distance, a single stage of the lens barrel is longer than a total length of the optical system in which airspaces between adjacent lens units are narrowed, thereby making it difficult to reduce thickness of a camera. When the lens barrel is composed of four or more stages, the lens barrel has a large maximum diameter which is disadvantageous for compact configuration of a camera.
As conventional examples which are configured for correcting the defects described above, there are known zoom optical systems disclosed by Japanese Patents Kokai Publication No. Hei 6-214157 and Kokai Publication No. Hei 6-214158 each of which comprises four positive, positive, positive and negative lens units movable at magnification change stages for obtaining a vari-focal ratio of 3 or higher and a compact design, and is configured to reduce aberrations to be produced by changing the magnification, zoom optical systems disclosed by Japanese Patents Kokai Publication No. Hei 8-122640 and Kokai Publication No. Hei 9-101457 each of which is composed of four positive, negative, positive and negative lens units, and zoom optical systems disclosed by Japanese Patents Kokai Publication No. Hei 9-15499 and No. Hei 9-15500 each of which is composed of four negative, positive, positive and negative lens units.
In each of the optical systems disclosed by Japanese Patents Kokai Publication No. Hei 6-214157, No. Hei 6-214158, No. 8-122640 and No. Hei 9-101457 out of the conventional examples mentioned above, the fourth lens unit is composed of two or more lens elements and has a composition wherein a lens element or lens elements are disposed on the object side of a negative lens element which has a deep concave surface on the object side and is disposed on the image side.
Accordingly, an axial distance as measured from a surface of incidence to a surface of emergence of the fourth lens unit, i.e., compositional length of the fourth lens unit is long and inadequate for shortening a compositional length of a lens barrel in its collapsed condition. That is, the optical system has a composition such as that shown in FIG. 1 and is not preferable for shortening a compositional length of a lens barrel in its collapsed condition. In FIG. 1, reference symbols G1, G2, G3 and G4 represent the first, second, third and fourth lens units, a reference symbol S designates an aperture stop, and reference symbols L.sub.P and L.sub.N denote a positive lens element and a negative lens element.
Further, the optical system disclosed by Japanese Patent Kokai Publication No. Hei 9-15499 has a field angle of incidence 2.omega. not narrower than 65.degree. but a low vari-focal ratio on the order of 2.9. Furthermore, the optical system disclosed by Japanese Patent Kokai Publication No. Hei 9-15500 has a field angle of incidence 2.omega. not narrower than 65.degree. and a vari-focal ratio on the order of 3.8, but allows an aperture stop to move at a magnification change stage, whereby the optical system comprises a large number of parts which are moved for changing a magnification, uses a lens barrel having a complicated structure or a large number of parts and requires a high manufacturing cost.
Moreover, each of the optical systems disclosed by Japanese Patents Kokai Publication No. Hei 9-15499 and No. 9-15500 is a zoom optical system composed of the four negative, positive, positive and negative lens units which are composed of lens elements in a relatively small number of seven, but uses a front lens element having a large diameter due to a fact that an aperture stop is disposed in the second lens unit which is disposed with a relatively wide airspace from the first lens unit. Since this optical system is focused onto an object located at a short distance by moving the third lens unit toward the object side, this optical system remarkably aggravates spherical aberration at its tele position in particular and hardly allows to shorten the distance to the object. Spherical aberration is remarkably aggravated since a converging light bundle which has passed through the second positive lens unit is incident onto the third lens unit and rays pass through high portions of a first negative lens element of the third lens unit in a condition where the third lens unit is moved toward the object side for focusing, and this aggravation is more remarkable at the tele position in particular.
In addition, it is strongly demanded to configure cameras more compact and problems are posed to dispose relatively large areas of lens surfaces which are disposed near a film surface and through which effective rays do not pass.